Field of the Invention
The invention concerns a method, a magnetic resonance apparatus and an electronically readable data storage medium for acquiring measurement data from a subject under examination by operation of a magnetic scanner, wherein the magnetization in at least two sub-volumes of a subject under examination to be examined is simultaneously manipulated and/or used to acquire the measurement data by execution of a sub-sequence.
Description of the Prior Art
Magnetic resonance (MR) technology is a known modality that can be used to generate images of the inside of a subject under examination. In simple terms, this is done by placing the subject under examination in a magnetic resonance scanner in a strong, static, homogeneous main magnetic field, also called the B0 field, at field strengths of 0.2 Tesla to 7 Tesla and higher, with the result that the nuclear spins of the subject are oriented along the basic magnetic field. In order to induce nuclear spin resonances, the subject under examination is exposed to pulses of radio frequency radiation (RF pulses), e.g. for the purpose of excitation or refocusing. The signal that results from the induced nuclear spin resonances are detected (received) and entered into a memory in a format known as k-space data. The k-space data are used as the basis for reconstructing MR images or obtaining spectroscopic data. Rapidly switched magnetic gradient fields are superimposed on the basic magnetic field for spatial encoding of the measurement data. The recorded measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix, populated with values, using e.g. a multidimensional Fourier transform.
All these technical subsystems, e.g. for controlling the gradient and for controlling the RF transmit/receive process, must be suitably addressed in a coordinated manner by a control computer. The control computer must also make the settings and switching operations needed for a specific imaging process for each of the subsystems, each at the correct time. Usually the volume to be imaged in an imaging procedure is captured in sub-volumes, for instance in a number of slices for 2D imaging or in a number of “slabs” for 3D imaging. The sub-volumes captured in this way are then combined to form a total volume. Sub-volumes may also be defined, for example, by “Regions of Interest” (ROI) or even “Volumes of Interest” (VOI), which can be defined specifically by the operator. Moreover, defining local saturation regions or local preparation or labeling pulses in magnetic resonance systems results in additional sub-volumes.
For this purpose, sequence control data, usually based on what is known as a measurement protocol, can be entered into the control computer for coordinated control. The sequence control data define various functional sub-sequences of a complete measurement sequence. For a magnetic resonance acquisition, a first sub-sequence may be, for example, a pulse sequence that locally achieves saturation of particular spins in a specific region. Further sub-sequences may contain, for instance, specific preparation pulses, and yet further sub-sequences are used for successive excitation and for receiving the magnetic resonance signals in different slices or slabs.
Clinical diagnostics require MR imaging techniques and MR spectroscopy techniques that allow maximum acquisition quality in minimum time. Unfortunately, these two requirements are often mutually exclusive, because short acquisition times usually involve compromises over the achievable quality of the measurement data.
In order to reduce the measurement times, MR measurement techniques in which the spins are excited in a number of sub-volumes, e.g. in a number of slices, simultaneously or in quick succession, and the signals thus generated are subsequently captured simultaneously or at short time intervals, have recently experienced a renaissance (by combining with parallel imaging techniques). These techniques, also called slice multiplexing techniques or simultaneous multislice techniques, are characterized by, at least during a time segment of the measurement, the magnetization in two regions, which are spatially separate at least in part, is simultaneously manipulated and/or used for the measurement data acquisition process in a targeted manner. Unlike these techniques, in conventional, established “multislice imaging”, the signal is acquired from at least two slices alternately, i.e. fully independently from one another, with a correspondingly longer measurement time.
In simultaneous multislice techniques, the captured signal is associated with the individual slices using k-space based parallel imaging algorithms, for example, (e.g. GRAPPA, “Generalized Autocalibrating Partially Parallel Acquisition”), which exploit the spatial receive profiles of at least two receive coil elements. This approach can be used to image a number of slices—for instance two or three or more—in the same time that would otherwise be taken to capture only one slice. For just a few simultaneously captured slices, the noise additionally induced in the acquired measurement data by the reconstruction algorithms, which is characterized by what is known as the g-factor, is practically negligible. Other approaches, such as Hadamard encoding, broadband data acquisition or simultaneous echo refocusing are known, for example, from the following publications:
Hadamard encoding (e.g. Souza et al., J. CAT 12:1026 (1988)): Two (or more) slices are excited simultaneously; a defined signal phase is applied to each slice by suitable design of the RF excitation pulses. The magnetization signal is received simultaneously from both slices. A similar second excitation of both slices is performed but with a modified relative signal phase in the slices. The rest of the imaging process (phase encoding steps) is performed as usual, and the method can be combined with any acquisition techniques ((multiple) gradient echo, (multiple) spin echo etc.). From the two acquisitions it is possible to separate the signal information from the two slices using suitable processing operations.
Broadband data acquisition (e.g. Wu et al., Proc. ISMRM 2009:2768): Two (or more) slices are excited simultaneously. The magnetization signal is received simultaneously from both slices. While data is being received, a gradient is activated along the slice normals, which results in separation of the signals from both slices in the frequency domain. The rest of the imaging process (phase encoding steps) is performed as usual, and the method can be combined with any acquisition techniques ((multiple) gradient echo, (multiple) spin echo etc.). From the simultaneously acquired data, it is possible to separate the signals from the two slices using suitable filtering.
Simultaneous echo refocusing (SER, SIR, e.g. Feinberg et al., MRM 48:1 (2002)): Two (or more) slices are excited in quick succession; a defined spatial dephasing is applied to each layer by suitable gradient pulses. The signal from the simultaneously manipulated magnetization is received from both slices at a short time interval apart using suitable gradient circuits. The rest of the imaging process (phase encoding steps) is performed as usual, and the method can be combined with any acquisition techniques ((multiple) gradient echo, (multiple) spin echo etc.). Images of the two slices can be generated as usual from the separately acquired data.
Magnetic resonance-based techniques, both tomographic imaging (MRI, magnetic resonance imaging) and spectroscopy (MRS, magnetic resonance spectroscopy), in general need “benign” underlying physical conditions to ensure optimum possible quality of the acquired data. For example, this applies to at least one of the criteria comprising spatial homogeneity, stability over time and absolute precision of the magnetic fields relevant to MR techniques (B0, the stationary main magnetic field, and B1, the alternating RF magnetic field).
Known measures that can be used to at least partially correct deviations from ideal underlying conditions include both system-specific and subject-specific adjustments. System-specific adjustments seek to correct the actual conditions associated with the MR system used, for instance conditions such as eddy-current induced dynamic field disturbances or gradient sensitivities. Adjustments specific to the subject under examination attempt to correct changes caused by the subject under examination, for instance a patient, introduced into the measurement volume of the MR system, such as susceptibility-related static field disturbances or spatial variations in the RF field.
Such methods for improving the quality of the acquired measurement data in particular by adjustments to the relevant measurement parameters, also for dynamic adjustment of the parameters, have undergone further development in recent years.
DE 10 2009 020 661 B4, for example, describes a method that is used to adapt parameters of a measurement sequence, e.g. in magnetic resonance technology, while the measurement sequence is running. This document also already describes that different functional sub-sequences are normally assigned different effective volumes. In other words, a different sub-volume of the total measurement volume is relevant to each sub-sequence.
It is also possible to adapt measurement parameters while a measurement is running in order to optimize the image quality. The fundamental idea of such dynamic adjustment is to configure the underlying physical conditions such that, preferably at any given time, they are as ideal as possible for the sub-volume that is currently relevant in the measurement process. If during an MR measurement, for example, the spins of a slice are excited and then the generated signal is detected, for this time segment of the MR measurement, the measurement parameters can be optimized to the region defined by the slice. In a subsequent excitation and detection of the next slice, the optimization can then be dynamically adapted accordingly, and so on.
The adjustable measurement parameters include, for example, the modulation frequency of the emitted RF pulses, the demodulation frequency of the received MR signal, scaling factors for the RF pulse amplitude, amplitude distribution and phase distribution of the RF currents on a plurality of transmit elements (if present). As a result of the improved local underlying conditions at any given time, said dynamic adjustments allow significant improvements in the image quality compared with static adjustment settings.
Conventionally, applying said dynamic adjustment methods specifically in combination with the above-mentioned simultaneous multislice techniques is possible only to a limited extent. The image quality improvement achieved by dynamic adjustments turns out to be higher, the smaller the relevant sub-volume in which the measurement parameters are adapted with the aim of achieving optimum conditions. If two or more spatially separate sub-volumes are excited simultaneously, as is the case in simultaneous multislice techniques, then according to known techniques, the relevant volume in which the underlying conditions are meant to be optimized must be chosen to be of a size that encompasses all the sub-volumes. The image quality improvement hence proves to be significantly lower as a result of the adjustment in such a large relevant volume.